Preheat your oven to 375°F (190°C). Gather your ingredients: lasagna sheets, thawed and drained spinach, ricotta, mozzarella, Parmesan, and marinara sauce.

In a large mixing bowl, combine the ricotta cheese with the drained spinach. Season generously with salt and pepper, and stir until well-blended.

Spread a layer of marinara sauce evenly across the bottom of a 9x13-inch baking dish.

Place a layer of lasagna sheets over the marinara sauce, followed by a portion of the ricotta-spinach mix, mozzarella cheese, and another drizzle of marinara sauce.

Continue layering by repeating the process until all ingredients are used, ending with marinara sauce and mozzarella on top.

Cover the baking dish tightly with aluminum foil and bake in the preheated oven for about 30 minutes.

After 30 minutes, carefully remove the foil and bake for an additional 10 minutes until the cheese is bubbly and golden brown.

Once baking is complete, remove the lasagna from the oven and let it cool for 10-15 minutes before slicing.